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Fundamentals of Flight
FM 1-203
LIFT ACTING ON AN AIRFOIL
This force is the component of the total aerodynamic force that acts at right angles to the resultant relative wind.
Resultant relative wind is the direction of the airflow with respect to the airfoil. This lift force is considered to act
through the center of pressure of the airfoil. The magnitude of this lift varies proportionally with the square of the
velocity (v2) of the airflow across the airfoil, air density, shape and size of the airfoil, and angle of attack.
TOTAL AERODYNAMIC FORCE
As air flows around an airfoil, a pressure differential develops between the upper and lower surfaces. The
differential, combined with the resistance of the air to the passage of the airfoil, creates a force on the airfoil. TAF
acts at the center of pressure on the airfoil and is normally inclined up and to the rear.
DRAG CONSIDERATIONS
The force that retards the motion of an aircraft or airfoil through the air is referred to as drag. At times, this force
decreases the performance of the aircraft and is a hindrance. At other times, drag can be an advantage. Because
drag retards motion and increases fuel consumption, it also affects performance objectives such as range, endurance,
and maximum velocity. To reduce drag, aviators will fly with clean aircraft. Drag requirements vary according to
flight conditions; therefore, aviators must understand this force to obtain the required performance from their
aircraft.
Types of Drag
Two types of drag are produced at subsonic speeds – parasite and induced. The drag acting on nonliftproducing surfaces of the helicopter is called parasite drag; the parasite drag that acts on helicopter rotor
systems is usually referred to as profile drag. At subsonic velocities, total drag is the sum of parasite and
induced drag.
Parasite Drag
Parasite drag results from the way the air flows around the aircraft. The shape, surface, smoothness, size,
and design of the aircraft affect this type of drag. Also contributing to parasite drag are add-ons such as
wingstores, exposed weapon systems, and drop tanks. If any of these cause turbulence or hinder airflow
about the aircraft, they increase the parasite drag.
Profile drag (helicopter only) is incurred from the frictional resistance (form and skin friction) of the rotor
blades passing through the air. While profile drag does not change significantly with the angle of attack of
the airfoil section, it does increase moderately as airspeed increases. Profile drag is the parasitic drag of the
rotor system.
Induced Drag
Induced drag results from the downward velocities imparted to the air by the wing as it produces lift and to
the vortexes developed by the wing or blade tip. If no lift is produced, there is no induced drag.
Total Drag
Total drag is that component of the total aerodynamic force parallel to the relative wind that tends to retard
the motion of the aircraft or airfoil. It acts parallel to the flight path or the relative wind but is not
necessarily parallel to the thrust vector.
TRANSLATING TENDENCY
During hovering flight, the single-rotor helicopter tends to drift laterally to the right. The tendency results from the
thrust exerted to the right by the tail rotor to compensate for main rotor torque. The aviator may prevent this right
lateral drift of the helicopter by tilting the main rotor disk to the left. This lateral tilt results in a main rotor force to
the left that compensates for tail rotor thrust to the right.
Compensating for Translating Tendency
Helicopter design usually includes at least one feature that helps the aviator compensate for this translating
tendency. One or more of the following features may be included:
(1) Flight-control rigging may be designed so that the rotor disk is tilted to the left when the cyclic control
is centered.
(2) The main transmission may be mounted so that the mast is tilted slightly to the left when the
helicopter fuselage is laterally level.
(3) The collective pitch control system may be designed so that the rotor disk tilts slightly left when
collective pitch is increased to hover the aircraft.
AIRFLOW IN FORWARD FLIGHT
a.
b.
c.
d.
e.
f.
g.
h.
In forward flight, air flows opposite the flight path of the aircraft. The velocity of the flow of air equals the
forward speed of the helicopter. Because the blades of the helicopter turn in a circular pattern, the velocity
of the airflow across a blade depends on the position of the blade in the rotor disk at a given instant, its
rotational velocity, and the airspeed of the helicopter.
The airflow meeting each blade continually varies as the blade rotates. Highest velocity of airflow occurs
over the right side of the helicopter. Airflow decreases to rotational velocity over the nose; the decrease
continues until the lowest velocity of airflow is reached over the left side of the helicopter. At that point,
the velocity of the airflow starts to increase, reaching rotational velocity over the tail and maximum
velocity over the right side.
The blade over the right side of the helicopter moves in the same direction as the helicopter. The velocity
of the air meeting this advancing blade equals the rotational velocity of the blade plus the velocity of wind
resulting from forward airspeed (aircraft airspeed).
The blade over the left side of the helicopter moves in a flow of air that is moving in the same direction as
the blade. The velocity of the airflow meeting this retreating blade equals the rotational velocity of the
blade minus forward airspeed (aircraft airspeed).
The blades over the nose and tail move essentially at right angles to the airflow created by forward
airspeed. Therefore, the velocity of the airflow meeting these blades equals the rotational velocity.
Change in airflow across the advancing and retreating blades causes a number of changes in the rotor
system. Forward airspeed is added to the rotational velocity at all points along the span of the advancing
blade to determine the wind velocity at any given point when the blade is at the 3 o’clock position.
Forward airspeed is subtracted from all points along the span of the retreating blade to determine the wind
velocity at any given point along its span when the blade is at the 9 o’clock position.
The airflow across the hub of the rotor system equals forward airspeed. This differential airflow velocity
across the rotor system causes the aerodynamic forces to vary along the blade span.
BLADE AREAS IN FORWARD FLIGHT
No Lift Areas:
(1) Reverse Flow. At the root of the retreating blade is an area where the air flows backward from the
trailing edge to the leading edge of the blade. This is because wind created by forward airspeed has a
higher velocity than the rotational velocity in this part of the blade span. The reverse flow area extends
from the mast to a point along the blade span where the rotational velocity is equal to the airspeed of
the helicopter. No positive lift is produced by this portion of the blade.
(2) Negative stall. In the negative stall area, rotational velocity exceeds forward flight velocity, causing
the resultant relative wind to move toward the leading edge of the blade. However, rotational velocity,
induced flow, and flapping action do not increase the angle of attack above the critical angel of attack.
Therefore, this blade region is operating with the resultant RW so far above the chord line that a
negative stall angle of attack results.
(3) Negative lift. In the negative lift area, rotational velocity, induced flow, and blade flapping combine to
reduce the angle of attack from a negative stall to an angle of attack that causes the blade to produce
negative lift. In this area, the resultant relative wind is still striking the airfoil above the chord line, but
the angle of attack has been reduced below the critical angle of attack. The air now has a greater
distance to flow along the underside of the airfoil. This results in the highest velocity and lowest
pressure on the underside of the airfoil. Te resulting lift force vector in this blade region is pointing
down, so this blade region is producing negative lift.
Positive Lift and Positive Stall:
(1) Positive lift is produced by the blade outboard from the three no-lift areas. In the positive lift regions,
the resultant relative wind is striking the airfoil below the chord line, resulting in a positive angle of
attack. The air now has a greater distance to flow over the airfoil than it has to flow beneath it.
Because the highest velocity and lowest pressure are on the top surface of the airfoil, the lift force is
oriented up. This mean the airfoil is producing positive lift.
(2) Under certain conditions, it is possible to have a positive stall area near the blade tip.
DISSYMMETRY OF LIFT
Cause of Dissymmetry of Lift:
(1) In forward flight, the combined effects of the differential airflow across the advancing and retreating
blades and the three no-lift areas on the retreating blade result in a dissymmetry of lift potential
between the advancing and retreating halves of the rotor disk.
(2) At an aircraft airspeed of 120 knots (200 feet per second), a 400-foot-per-second blade speed
differential exists between the advancing and retreating blades. Because lift increases with the square
of the velocity, a potential lift variation exists between the advancing and retreating sides of the rotor
disk. If the aviator fails to compensate for this lift differential, the helicopter becomes uncontrollable.
(3) In forward flight, two factors (air density and blade area) are the same for the advancing and the
retreating blades. The airfoil shape is fixed for a given blade. The only remaining variables are blade
speed and blade angle of attack. These two variables must compensate for each other during forward
flight to overcome dissymmetry of lift. Two factors (rotor RPM and aircraft airspeed) control blade
speed during flight. Both can be varied to some degree; however, they must remain within the
operating limits specified for the aircraft. Because blade speed is relative constant, angle of attack
remains the one variable that can compensate for dissymmetry of lift. Through cyclic feathering, the
pitch angle of the rotor blades can be varied throughout their range, from flat to stalling, to change the
angle of attack and to compensate for lift differential. Blade flapping also causes a change in the angle
of attack that helps to compensate for dissymmetry of lift.
Compensating for Dissymmetry of Lift:
a.
Blade flapping is the up and down movement of the rotor blade about a flapping hinge. Flapping alone
or in conjunction with the cyclic feathering can eliminate dissymmetry of lift and allow the pilot to
maneuver the helicopter. The upflapping of the blade over the nose and downflapping over the tail of
the helicopter cause the tip path plane to tilt to the rear. This rearward tilt of the rotor disk is called
blowback.
1.
2.
3.
4.
b.
The upflapping and downflapping of the rotor blades aerodynamically change the angle of
attack and, therefore, the amount of lift the blade produces.
The upflapping blade causes air to move downward through the rotor blades. This has the
same effect as an increase in the induced flow velocity, which decrease the angle of attack.
The upflapping of the blade reduces the angle of attack just enough to cancel any increase in
the lift expected because of the increased air velocity across the blade.
The downflapping blade causes air to move upward through the rotor blades. This has the
same effect as reducing the induced flow velocity and results in an increase in the angle of
attack. The downflapping of the blade increases the angle of attack just enough to cancel any
loss of lift expected because of decreased airflow across the blade.
Upflapping and downflapping do not change the amount of lift produced by the rotor system.
The blades flap to equilibrium. However, flapping changes the attitude of the rotor system
(blowback) and, therefore, the direction of the total lift vector. This reduces helicopter
airspeed.
Cyclic feathering. The aerodynamic flapping of the rotor blades as they compensate for dissymmetry
of lift changes the attitude of the rotor disk; this directs the total thrust (lift) vector to the rear, which
changes airspeed. The aviator must then be able to control the attitude of the rotor system and the
direction of the total thrust vector. Cyclic feathering changes the angle of incidence differentially
around the rotor system. This cyclic stick movement decreases the angle of incidence by the same
amount 180 degrees of travel later.
Tail Rotor Dissymmetry of Lift:
The tail rotor experiences dissymmetry of lift during forward flight because it also has advancing and
retreating blades. Dissymmetry is corrected by a flapping action. The TH-67 incorporates a
Delta hinge. The delta hinge is not oriented parallel to the blade chord. It is designed so flapping
automatically causes the blades to feather, which corrects for dissymmetry of lift.
TRANSVERSE FLOW EFFECT
As the forward velocity of the helicopter increases, another phenomenon of differential airflow in the rotor system
occurs. While dissymmetry of lift involves the advancing and retreating sides of the rotor disk, transverse flow
effect involves the front and rear halves of the rotor disk. Because of coning and the forward tilt of the rotor system,
there is a differential airflow across the front and rear halves of the rotor disk. This is the transverse flow effect. The
pilot can recognize the transverse flow effect because of increased vibrations of the helicopter at airspeeds just
below EO on takeoff and after passing through ETL during landing. These vibrations take place because the
greatest lift differential between the fore and aft portions of the rotor system occurs at those airspeeds. The
vibrations are caused by increased induced drag on the blades as they pass over the tail of the helicopter.
Causes of Transverse Flow Effect:
a.
b.
c.
Air moving across the rotor disk in forward flight is deflected downward because of induced
flow. The greater the distance air must flow over the rotor disk, the longer the disk has to act
on it and the greater the deflection. This results in a more horizontal flow of air over the
forward half of the rotor disk than over the rear half; thus there is less induced flow over the
front half of the rotor disk than over the rear half.
Greater induced flow velocity through the rear half of the disk decreases the angle of attack.
Because the blades are operating at the same angle of incidence over the nose and tail,
reducing the induced flow over the nose causes an increase in the angle of attack.
An increased angle of attack in the front half of the rotor disk increases the lift of the blade at
this location. Increased lift on the blade over the nose causes the blade to flap up. Because of
phase lag, the maximum upflapping blade displacement occurs over the left side of the
helicopter. The decreased lift produced by the blade over the tail, combined with phase lag,
results in maximum dowflapping blade displacement over the right side of the helicopter. The
displacement tilts the rotor disk to the right, changing the direction of the thrust vector. This
change in the attitude of the rotor disk must be prevented if the helicopter is to maintain a
straight flight path.
Compensating for Transverse Flow Effect:
A left cyclic input decreases the pitch angle and angle of attack of the blade over the nose while increasing
the pitch angle and angle of attack of the blade over the tail. These changes to blade angles of attack cause
changes to lift. As the pilot senses the right tilt of the rotor, he must apply left cyclic to prevent a change in
the attitude of the disk. As forward speed increases, the potential lift differential between the fore and aft
portions also increases. Additional left cyclic inputs are required to prevent the right tilt of the rotor as a
result of transverse flow effect. At higher airspeeds, lift differential between the fore and aft portions of the
disk begins to decrease. The cyclic stick must be moved back to the right at higher cruise speeds.
AIRFLOW DURING HOVERING
At a hover, the blade-tip vortex (air swirl at the tip of the rotor blades) reduces the effectiveness of the outer blade
portions. The vortex of a preceding blade also severely affects the lift of the following blades. If the vortex made
by one passing blade remains swirling for a number of seconds, then two blades operating at 350 RPM create 700
long-lasting vortex patterns per minute. The continuous creation of new vortexes and the continuous ingestion of
existing vortexes are the primary causes of the high power required to hover.
1.
Ground effect is a condition of improved performance encountered when the aircraft is operating near
the ground. It is due to interference of the surface with the airflow pattern of the rotor system and is
more pronounced closer to the ground. The high power requirement needed to hover OGE is reduced
when the aircraft is operating in-ground effect.
2.
A helicopter hovering out-of-ground effect requires a great deal of power and high blade pitch angles
to move a tremendous quantity of air at high velocities down through the rotor system to produce lift
equal to weight. The velocity of this downwash continues to increase until it reaches maximum
velocity, about one rotor disk below the plane of rotation. This results in a large induced-flow
velocity. The resultant relative wind produces a lift vector that is inclined well to the rear.
3.
Increased blade efficiency during the IGE is due to two phenomena.
a. Reduction of the velocity of the induced airflow. Because the ground interrupts the airflow
under the helicopter, the entire flow is altered. This reduces downward velocity of the
induced flow. The result is less induced drag and amore vertical lift vector. The lift needed
to sustain a hover IGE can be produced with less power because the lift vector is more
vertical.
b. Reduction of the rotor-tip vortex. When the helicopter is IGE, the downward and outward
airflow pattern tends to restrict vortex generation. Thus the outboard portion of the rotor
blade becomes more efficient, reducing overall system turbulence caused by ingestion and
recirculation of the vortex swirls.
4.
5.
6.
7.
To move from an OGE hover to an IGE hover, the aviator must first reduce the total lift of the rotor
system by reducing collective pitch. As the helicopter moves nearer the surface, the ground interrupts
downward flow; the air then moves horizontally outward from under the rotor disk. This prevents high
induced-flow velocities from developing below the rotor and is manifested as a reduced induced-flow
velocity through the rotor disk. Because of the decreased blade pitch angle and reduced induced-flow
velocities, the lift vector is not tilted as far to the rear. Less power is required to produce the same
amount of lift. Also, the horizontal flow of air nearer the plane of rotation tends to restrict the
formation of blade-tip vortexes.
Rotor efficiency is increased by ground effect to a height of about one rotor diameter for most
helicopters. At a rotor height of one-half rotor diameter, thrust increases about 7 percent. At rotor
heights above one rotor diameter, thrust increase is small, decreasing to zero at a height of about 1
¼ rotor diameter.
Maximum ground effect occurs when aircraft hover over smooth, paved surfaces. Over tall grass,
rough terrain, revetments, or water, ground effect may be seriously reduced. This phenomenon is due
to the partial breakdown and cancellation of ground effect and the return of large vortex patterns with
increased downwash velocities.
If a helicopter hovering OGE descends into a ground-effect hover, blade efficiency increases because
of the more favorable induced flow. As efficiency of the rotor system increases, the aviator reduces
the blade pitch angle to remain in the ground-effect hover. Hovering in-ground effect requires less
power than hovering out-of-ground effect.
TRANSLATIONAL FLIGHT
Each knot of incoming wind gained by horizontal movement or surface wind improves the efficiency of the rotor
system. As the incoming wind enters the rotor system, turbulence and vortexes are left behind and the flow of air
becomes more horizontal. Therefore, aircraft performance improves. Improved rotor efficiency resulting form
directional flight is called translational lift. When a single-rotor helicopter makes the transition from hover to
forward flight, the tail rotor becomes more aerodynamically efficient. The tail rotor works in progressively less
turbulent air as speed increases. As tail-rotor efficiency improves, more thrust is produced. This causes the aircraft
nose to yaw left if the main rotor turns counterclockwise. During a takeoff, where power is constant, the aviator
must apply right pedal as speed increases to correct for the left-yaw tendency.
a.
Airflow Pattern, 1 to 5 knots. In the airflow pattern for a forward speed of 1 to 5 knots, the downwind
vortex is beginning to dissipate; induced flow down through the rear of the rotor disk is more
horizontal than at a hover.
b.
Airflow Pattern, 6 to 15 knots. The airflow pattern at a speed of 6 to 15 knots. Airflow is much more
horizontal than at a hover. The forward edge of the downwash pattern moves closer to the helicopter
as airspeed increases. At 10 knots, the forward edge of the downwash pattern has moved to a position
under the blade tips over the nose. At 15 knots, this downwash pattern moves to a position toward the
tail of the aircraft. This allows the flow of air through the rotor system to become more horizontal and
streamlined as airspeed increases.
NOTE: As the helicopter makes the transition from a hover to directional flight, dissymmetry of lift and
the transverse flow effect begin to affect the rotor system.
EFFECTIVE TRANSLATIONAL LIFT
a.
At about 16 to 24 knots (depending on the size, blade area, and RPM of the rotor system), the rotor
completely outruns the recirculation of old vortexes and begins to work in relatively undisturbed air.
The rotor no longer pumps the air in a circular pattern but continually flies into undisturbed air. The
flow of air passing through the rotor system is more horizontal, depending on forward speed. Induced
flow is reduced; therefore, induced drag is reduced. This increases the angle of attack.
b.
As single-rotor aircraft speed increases, translational lift becomes more effective; the nose rises or
pitches up and rolls to the right. This tendency is caused by the combined effects of dissymmetry of lift
and the transverse flow effect. Dissymmetry of lift causes blowback of the rotor system; the transverse
flow effect tilts the rotor to the right. Aviators must correct for this tendency to maintain a constant
rotor-disk attitude that moves the helicopter through the speed range where ETL occurs.
RETREATING BLADE STALL
The retreating blade of helicopters tends to stall in forward flight. Just as the stall of an airplane wing limits the lowspeed possibilities of the airplane, the stall of a rotor blade limits the high-speed potential of a helicopter. The speed
of the retreating blade (the blade moving away fro the direction of flight) decreases as forward speed increases.
The smaller area of the retreating blade, with its high angles of attack, however, must still produce an amount of lift
equal to that of the larger area of the advancing blade, its low angles of attack. As the speed of the retreating blade
decreases with forward speed, the blade angle of attack must be increased to equalize lift throughout the rotor-disk
area. If this angle increase is continued, the blade stalls at some high forward speed. Tip stall causes vibration and
buffeting at critical airspeeds. If the blade descends, causing greater angle s of attack, the stall spreads inboard; the
helicopter may pitch up and roll left.
Causes and Effects of Retreating Blade Stall:
a.
b.
c.
As forward airspeed increases, the no-lift areas move left of center, covering more of the
retreating blade sectors. This requires more lift at the out retreating blade portions to
compensate for the loss of lift of the inboard retreating sections. In the area of reversed flow,
the rotational velocity of this blade section is slower than the aircraft airspeed. Therefore, the
air flows from the trailing to leading edge of the airfoil. In the negative stall area, the
rotational velocity of the airfoil is faster than aircraft airspeed; air flows from the leading to
trailing edge of the blade. However, the resultant relative wind strikes the blade so far above
the chord line that the airfoil is operating with a negative angle of attack. The angle of attack
is above the critical angle; therefore, the blade stalls. In the negative-lift area, rotational
velocity and blade flapping have reduced the angle of attack below the critical angle.
However, the resultant relative wind is still striking far enough above the airfoil to create
negative lift.
The loss of disk area on the retreating half of the rotor disk, resulting from the increasing size
of the no-lift areas, causes the blade to flap down. This downflapping increases the angle of
attack. Maximum flapping velocity will occur at the blade tip. Because of this increase in
angle of attack caused by flapping, coupled with higher blade pitch angles, the angle of attack
at the tip of the retreating blade may exceed the critical angle of attack.
In a single rotor helicopter upon entry into blade stall, the first effect is generally a noticeable
vibration. This may be followed by a left roll and a tendency for the nose to pitch up. The
tendency to pitch up and roll left may be relatively insignificant for helicopters with semirigid rotor systems because of pendular action. If the cyclic stick is held forward and
collective pitch is not reduced or is increased, this condition worsens. The vibration greatly
increases; control can be lost. By being familiar with the conditions that lead to blade stall,
the aviator can recognize and correct them. In single rotor helicopters, warnings of
approaching retreating blade stall are as follows:
1.
2.
3.
Abnormal vibration
Pitch up of the nose
Tendency of the helicopter to roll in the direction of the stalled side
Conditions Likely to Produce Blade Stall:
The following conditions are most likely to produce blade stall:
a.
b.
c.
d.
e.
High blade loading (high gross weight)
Low rotor RPM
High-density altitude
Steep or abrupt turns
Turbulent air
S (Steep)
T (Turbulence) A (Altitude)
L (Low rotor)
L (Loading)
Preventing Retreating Blade Stall:
a.
The aviator should take corrective action immediately when blade stall is likely and exercise
extreme caution when maneuvering. An abrupt maneuver, such as a steep turn or pull-up,
may result in dangerously severe blade stall. Aircraft control and structural limitations of the
helicopter would be threatened. Blade stall normally occurs at high airspeeds. To prevent
blade stall, the aviator must fly slower than normal when:
1.
2.
3.
b.
When blade stall is suspected, the aviator should follow the procedures outlined in the
operator’s manual. These steps will normally include:
1.
2.
3.
4.
5.
c.
Density altitude is much higher than standard
The aircraft is flying in a high-drag configuration; for example, if it is equipped with
floats, external stores, weapons, speakers, floodlights, or sling loads.
Air is turbulent.
Reducing power
Reducing airspeed
Reducing the severity of the maneuvering
Increasing RPM toward the upper limit
Checking pedal in trim
In severe blade stall, aircraft control may be lost. The corrective action is to follow
procedures outlined in the operator’s manual to shorten the duration of the stall and regain
control.
SETTLING WITH POWER
Settling with power is a condition of powered flight in which the helicopter settles in its own downwash. The
condition may also be referred to as the vortex ring state, which is one of the four flow state regions.
Conditions Conducive to Settling with Power:
a.
Conditions conducive to settling with power are:
1.
2.
3.
4.
b.
c.
Vertical or near-vertical descent of at least 300 feet per minute
Low forward airspeed
Rotor using 20-100% engine power
Insufficient power to retard the sink rate
Conditions can occur during downwind approaches, formation approaches/takeoffs, steep approaches,
NOE flight, mask/remask operations, and hover OGE.
Under the above conditions, the helicopters may descend at a higher rate, exceeding the normal
downward induced-flow rate of the inner blade sections. As a result, the airflow of the inner blade
sections is upward relative to the disk. This produces a secondary vortex ring in addition to the normal
tip-vortex system. The secondary vortex ring is generated about the point on the blade where the
d.
e.
f.
airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the
disk; rotor efficiency is lost even though power is still supplied from the engine.
Downward velocity is greatest at the blade tip where blade airspeed is highest. As blade airspeed
decreases nearer the disk center, downward velocity is les. The descent is so rapid that induced flow at
the inner portion of the blades is upward rather than downward. The upflow caused by the descent has
overcome the downflow produced by blade rotation. If the helicopter descends under these conditions,
with insufficient power to slow or stop the descent, it will enter the vortex ring state. During this
vortex ring state, roughness and loss of control occur because of the turbulent rotational flow on the
blades and the unsteady shifting of the flow along the blade span.
The vortex ring state can be completely avoided by descending on flight paths sallower than about 30
degrees (at any speed).
For steeper approaches, the vortex ring state can be avoided by using rates of descent versus horizontal
velocity either faster or slower than those passing through the area of severe turbulence and thrust
variation.
FLOW STATES IN DESCENDING FLIGHT
When a helicopter is descending, the airflow through the rotor system is different from what it was in level flight at
the same speed. The more vertical the descent, the greater the changes in airflow pattern. Four flow states are
generally recognized as occurring in descending flight:
a.
b.
c.
d.
Normal thrusting state
Vortex ring state
Autorotative state
Windmill brake state
Normal Thrusting State:
For hovering and at low rates of descent, the induced flow generated by the blades exceeds he rate of
descent. The airflow is down with respect to the rotor disk. There is a difference in induced-flow velocities
along the blade span. This results from varying rotational velocities and angles of attack from the root to
the tip of the blade. The thrust generated by the rotor system is quite steady, and engine power required to
maintain rotor RPM remains constant.
Vortex Ring State:
At greater rates of descent, the wind developed by the vertical velocity of the helicopter is opposing the
normal induced flow developed by the rotor. This upward flow of air cancels the induced flow and, in fact,
allows an upward flow of air in that part of the rotor disk where the upward velocity of flow exceeds the
induced flow velocity. This results in an unsteady turbulent flow of air through the rotor disk and
increasing rates of descent, even though additional power is supplied by the engine.
Autorotative State:
At rates of descent exceeding the vortex ring state, the autorotative state is achieved. At these higher rates
of descent, the turbulent, unsteady flow begins to smooth out. There is some rate of descent where no
engine power is required to maintain rotor RPM. The rotor system is extracting enough energy to provide
the power requirement from the air in this state of vertical flight. The autorotative state is the boundary
between conditions where engine power must be delivered to the rotor to prevent rotor RPM decay and
where power must be extracted from the rotor system to prevent rotor overspeed.
Windmill Brake State:
At very high rates of descent, the airflow is almost entirely up through the rotor system. The rotor system
is acting similar to a windmill. It is extracting more energy from the air than is required for flight. It is not
a normal operating state for the rotor system; some energy must be extracted to prevent a rotor overspeed.
This can usually be accomplished by increasing collective pitch, which adds more drag to the system.
DYNAMIC ROLLOVER
A helicopter is susceptible to a lateral rolling tendency called dynamic rollover. This dynamic rollover can occur on
level ground; however, it is more likely to occur and more hazardous during slope or crosswind landing and takeoff
maneuvers. Each helicopter has a critical rollover angle beyond which recovery is impossible. If the critical
rollover angle is exceeded, the helicopter will roll on its side regardless of the cyclic corrections made. The rate of
rolling motion is also critical. As the roll rate increases, the critical rollover angle at which recovery is still possible
is reduced. Depending on the type of helicopter, the critical rollover angle may change based on which skid or
wheel is touching the ground (acting as a pivot point), crosswind component, lateral offsets in CG, and left pedal
inputs for torque correction.
Characteristics:
a.
b.
c.
d.
e.
Dynamic rollover starts when the helicopter has only one skid or wheel on the ground. That gear may
become a pivot point for lateral roll. When this happens, lateral cyclic control response is more
sluggish and less effective than for a free-hovering helicopter. The gear may become a pivot point for
a variety of reasons. Most are aviator-induced. The gear or skid can become caught on objects
projecting form the landing surface such as a bent piece of steel planking; it can possibly become stuck
in soft asphalt or mud. Another way the gear becomes a pivot point is if the helicopter is forced into a
slope by a improper landing or takeoff technique. Whatever the cause, if the gear or skid becomes a
pivot point, dynamic rollover is possible when later aviator actions are incorrect.
The tail rotor may add to this rolling tendency if cyclic is not correctly applied to counteract lateral
tail-rotor thrust. A cross-wind can also contribute to rollover by causing sideward drift or by further
accentuating the aircraft bank angle needed to land on a slope.
A smooth, moderate collective-pitch change may be the most effective way to stop rolling motion.
Collective must not be changed so fast as to cause fuselage and rotor-blade contact. If a helicopter is
on a slope and the roll starts to the upslope side, reducing collective too fast can create a high roll rate
in the opposite direction. If the collective reduction causes the downslope gear or skid to hit the
ground abruptly, the rate of motion may cause a roll or pivot about the downslope gear.
Suddenly increasing collective pitch to become airborne may not stop dynamic rollover. If the gear or
skid that acts as a pivot point does not break free of the ground as collective is increased, the rollover
tendency will increase and worsen. If the skid or gear does break free of the round as collective is
increased, it can cause an abrupt rolling movement in the opposite direction because of the pendulum
effect. This movement can become uncontrollable.
When performing maneuvers with one skid or gear on the ground, the aviator should keep the
helicopter trimmed, especially laterally. Control is maintained if the aviator maintains trim, does not
allow lateral roll rates to become rapid, and keeps the bank angle from exceeding the critical rollover
angle for the helicopter. The aviator must takeoff smoothly with only small changes in pitch, roll, and
yaw. Untrimmed moments must be avoided.
Types of Motion:
a.
A downslope rolling motion is caused when the aviator applies too much cyclic into the slope. During
landings or takeoffs when the downslope skid is on the slope, the upslope skid may rise enough to
exceed lateral cyclic control limits. Thus a downslope rolling motion occurs.
b.
An upslope rolling motion is caused when the aviator applies cyclic into the slope in coordination with
collective-pitch application. During landings or takeoffs when the upslope skid is on the slope, the
downslope skid may rise enough to exceed lateral cyclic control limits. Thus an upslope rolling
motion occurs.
Preventing Upslope Rollover During Lift-off:
Upslope rollover characteristics are possible during lift-off. Upslope rollover results from applying too
much cyclic to hold the upslope gear against the slope. If collective pitch is improperly applied, the
aircraft then rapidly pivots around the longitudinal axis of the upslope landing gear to the point of rollover.
To prevent upslope rollover, the aviator needs to cautiously lift the downslope side of the helicopter to a
level position, simultaneously working the cyclic control to neutral. Once the cyclic is neutral and the
upslope landing gear has no side pressure applied, the aviator is cleared for a vertical lift-off to a hover and
then to a normal takeoff.
Preventing Downslope Rollover During Landing:
Downslope rollover is caused when the helicopter becomes tilted beyond the cyclic control limits by the
steepness of a slope. If the slope (wind or CG conditions) exceeds lateral cyclic-control limits, the mast
forces the rotor to tilt downslope. The resultant rotor lift has a downslope component, even with full
upslope cyclic applied. To prevent downslope rollover during land, the aviator slowly descends vertically
to a light ground contact the upslope gear. While observing lateral level reference frames, the aviator
pauses and maintains a positive-heading control. Then using careful collective pitch control, he slowly and
cautiously lowers the downslope gear. As the cyclic stick nears the lateral stop, he pauses to compare the
distance to go with the lateral control travel remaining (limits are given in the appropriate operator’s
manual). If it appears the cyclic will contact the upslope control, the aviator stops before the downslope
gear is firmly on the ground, returns the helicopter to a level attitude, and aborts the slope landing the
aviator lifts off and moves a few feet for another attempt on a lesser slope.
Preventing Downslope Rollover During Lift-off:
After landing inadvertently on an excessive slope, the aviator will attempt to lift off. If the upslope gear
tends to rise the aviator should smoothly lower the collective pitch. With full cyclic applied, however, the
resultant lift of the main rotor is not vertical or directed upslope enough to raise the downslope gear.
Therefore, if the upslope gear rises, the mast causes the resultant rotor lift to move farther downslope. This
increases the downslope roll tendency, which continues to increase with added collective pitch. The
corrective action is to reduce power at the first sign of a lateral roll around the downslope skid. Before
another lift-off is attempted, appropriate aviator action may be to:
a.
b.
c.
d.
Await different wind conditions
Change CG loading
Dig out from under the upslope gear
Notify operations to send a recovery crew